The present invention relates to aircraft engine systems and, more particularly, to a use of a chemical solution or abrasive flow to strip the topical layer from the inside of a hollow mechanical structure such as a hollow core airfoil of a high bypass turbofan engine.
Modern high bypass turbofan engines incorporate wide chord fan blades for improved aerodynamic efficiency. To minimize system weight, fan blade airfoils and dovetails are made hollow either by material removal, or by fabricating hollow constructions by forming and joining one or more pieces into a final assembly. Titanium alloys are typically used in front-end turbomachinery static and rotating airfoils.
Since many of the manufacturing processes involve exposing the structure to elevated temperatures for long periods of time, the Titanium oxidizes, or forms an alpha case structure on exposed surfaces. The high temperatures from the thermal processes cause formation of the alpha case layer which is hard and brittle, and can thereby lead to failures. While this condition can readily be removed on the exterior of the component, interior cavities are not accessible to traditional material removal or surface enhancement methods. The alpha case, or embrittled material layer, is known to have negative effects on material properties, particularly on fatigue strength, an attribute critical to rotating blades.
The joining and forming techniques used to produce hollow parts also contribute to the creation of many localized flaws and defects inside the hollow parts which cannot be removed once the part is fabricated. These flaws act as stress raisers which can effectively reduce the fatigue strength of the component, and can result from mating surface misalignment or offsets, edge damage prior to bonding, poor stop-off definition, contamination, or tearing due to excessive forming rates. Such conditions are undesirable and detrimental to the integrity of the blade or vane. Furthermore, fatigue crack initiation and propagation is a major cause of failure in turbomachinery flow path hardware. Achieving the highest possible fatigue strength of each part, particularly of critical rotating hardware, is crucial to the overall reliability and integrity of the machine.
It is therefore highly desirable and an object of the present invention to produce a final part void of internal alpha case and surface defects which degrade the fatigue properties of the structure.
Another object of the present invention is the use of an etchant, acid, or abrasive slurry on the interior of a fabricated part to overcome the disadvantages of the prior art.
Another object of the present invention is to allow venting and full cavity access in the interior of a fabricated part to maximize the use of the etchant, acid, or abrasive slurry on the interior of the part.
Another object of the present invention is to recontour local, sharp discontinuities that occur at bond lines.
These objects and other features and advantages will become more readily apparent in the following description when taken in conjunction with the appended drawings.